The invention relates to a pressure monitor, and more particularly a device for monitoring tire pressure in tires equipped with a pressure valve.
It is conventional for tire manufacturers to provide information on the most efficient tire use in order to extend the life of the tires, as well as improve gas mileage of cars, trucks and other vehicles. When the tire pressure drops below a normal level, gas mileage is adversely affected. If the high pressure is higher than the recommended limit, the tire may explode, causing property damage and possible injuries to people. Experienced drivers routinely check the tire pressure to make sure that it is within the recommended level.
The procedure usually involves the use of a pressure gauge that is manually connected to the valve of the tire and reading of the indicia on a pop-up pressure gauge stem. When the pressure is below the required level, the driver would add air into the tire to inflate it to the desired level. If the pressure exceeds the recommended limit, the tire might be deflated to reach the necessary internal pressure. Such a procedure is time consuming since every tire has to be checked for compliance with the manufacturer's instructions.
In order to simplify the procedure, the industry has developed various tire pressure indicators that are designed for attachment to the tire pressure valves for continuous monitoring of the status of the tire pressure. One such indicator is disclosed in U.S. Pat. No. 3,938,078 issued on Feb. 10, 1976 to Davis et al. In that patent, the unit is directly connectable to the valve stem of a car tire. When activated, the unit can be seen by the driver through suitably adjusted side mirrors. The device has a plunger that fits into and can open the tire pressure valve. A piston contacts a compression spring to close an electrical circuit (powered by a battery) and extinguish the light bulb. A tube filled with liquid is mounted between the plunger piston and the second piston. When fluid pressure from the tire is escaping, it moves the plunger piston against the second piston to operate the compression spring and close the circuit.
Another tire pressure indicator is disclosed in U.S. Pat. No. 4,059,823 issued on Nov. 22, 1977 to Martin et al. This device not only gives visual indication when the tire pressure is low by energizing a light bulb, but also provides a scale to indicate the extent of under- or over-inflation of the tire.
U.S. Pat. No. 4,726,223 issued on Feb. 23, 1988 to Huang provides for an alarm, audible or visual to give a signal when the tire is under inflated. U.S. Pat. No. 5,025,244 issued on Jun. 18, 1991 to Huang discloses a device that uses a piston with a mercury cell, with a positive end of the cell being in contact with the positive end of a light bulb. A pressure spring keeps the mercury cell in contact with the light bulb. A sliding member is placed under a lower end of the pressure spring. The tire pressure causes the spring to be either compressed or released to turn the light bulb on or off.
U.S. Pat. No. 5,289,161 issued on Feb. 22, 1994 to Huang discloses the use of a diaphragm that is positioned in the casing near an air inlet of the pressure gauge. The diaphragm moves in response to air pressure entering the casing from the tire. A spring-loaded push rod moves inside the casing in response to a movement from the diaphragm, which, in turn, activates signal switches mounted in the casing.
U.S. Pat. No. 5,557,256 issued on Sep. 17, 1996 to Achterholt shows a valve cap with a diaphragm mounted in an inner housing of the pressure chamber. A reference pressure chamber is used to compare with the tire pressure and, when the tire pressure drops below the predetermined level, to deflect the diaphragm. A permanent magnet is placed on the diaphragm. A hand-held pressure checking device also has a magnet, and when brought in proximity to the valve cap, indicates position of the magnet inside.
While these devices work satisfactorily under many conditions, some of them are too complicated and inefficient, while others are not well designed to withstand a continuous vibration associated with a moving vehicle. As a result, there exists a need for a simple, easy to use tire pressure monitor that can be easily calibrated to indicate low pressure or above the limit pressure in the tires.